The increased cost of energy from fossil fuel sources which occurred in the years following 1973 has prompted a search for more economical sources of energy. One source used from antiquity is wind. Unfortunately, wind has always had the disadvantage of extreme variability in the energy available from it. Particularly when one attempted to generate electricity from wind power this required some elaborate and expensive scheme for storing the wind energy in times of excess so that it could be used in times of deficiency. In the early designs of wind electric machines for example, this storage technique depended on a bank of lead-acid batteries which could be charged and discharged as the wind speed and demand for energy dictated.
A new concept in energy storage, in part mandated by Federal law, is to use the nation's power grid structure as the storage unit. Excess power generated by a privately owned generating unit can be fed into the power grid. The energy is not really "stored" in the grid, but instead serves to reduce the generating requirements at the utility-owned generating stations. The net use of electricity by the private owner of a wind electric generator can be monitored in the usual fashion, and the account credited or debited accordingly as the private system's owner supplies excess electricity to the grid or uses utility-generated electricity.
Wind generator units which are available now typically employ synchronous electric generation. This requires that the generator speed be controlled very accurately so as to keep the generator frequency and phase in close relationship with the power grid itself. Failure to do so can result in the equivalent of a short circuit between the power grid and the motor, which must be immediately detected by circuit breakers to prevent damage to the generator.
Although not generally realized, an induction generator (which is simply an induction motor) can be used to supply power to a power grid. The grid itself furnishes the necessary excitation current to establish the initial poles. Since an induction motor/generator has no salient poles, the exciting current establishes the poles properly to place the generator in phase with the power grid current. Thereafter, as the power input to the induction motor/generator varies, the slip changes and the poles shift magnetically to match the phase of the power grid current, with the added bonus of acceptable power factor as well. Induction generators are not as efficient in converting mechanical energy to electrical as are synchronous generators, and this is important in very large installations. These installations also can afford the capital costs involved in providing the necessary speed and phase control. But in a situation where the mechanical energy input is nearly free, the efficiency of the generator is not as important as the system capital cost and reliability.
The theory of induction generation has been known for many years. Two representative references, Alternating Current Machines, p. 208 ff., 1948, D. Van Nostrand, and Induction Generator Theory and Application, Barkle and Ferguson, AIEE Transactions, February 1954, each contain a useful discussion of the induction machine's application for use as a generator. Barkle et al. state that simply placing the induction generator on a power grid which has sufficient synchronous generating capacity is sufficient to automatically synchronize the induction generator with the power grid frequency and phase. Thus, the induction generator enjoys a substantial advantage over the synchronous generator with respect to the amount of control of phase and frequency required for successful operation. In fact, Barkle et al. show that the difference between an induction machine operating as a generator or as a motor is simply whether or not the output shaft is driving a load for motor operation or is being driven by a prime mover. If the machine is to operate as a generator, it must be driven at greater than its synchronous speed, i.e. with "negative" slip. FIG. 3 graphically illustrates this principle. When the machine's shaft speed is less than its synchronous speed identified by the 0 slip line labelled 600-900-1200, it functions as a motor and produces output torque in response to the input of electric power. When a prime mover drives the machine's shaft at greater than the synchronous speed, then electric flow power is out of the machine, and it functions as a generator.